Preparation process of multilayer polypropylene capacitor film

Abstract

The invention relates to the technical field of capacitor films. The invention discloses a preparation process of a multilayer polypropylene capacitance film, which comprises the following steps: S1, raw material preparation: preparing basic resin, a nano ceramic filler, a nucleating agent and an antioxidant into multilayer raw materials respectively; s2, fusing multiple layers of raw materials by an independent screw extruder, and compounding by a co-extrusion die head to form a film blank; and S3, carrying out air ring gradient cooling on the film blank. And S4, longitudinally and transversely stretching the film blank. And S5, performing heat setting in a segmented heating furnace, and performing corona treatment to improve surface energy. And S6, segmentally curing the moisture-proof coating by using an infrared oven. The multilayer heterostructure is adopted, the comprehensive performance of the capacitor film is improved through material matching, the dielectric constant, the breakdown strength and the moisture resistance are enhanced, and the problem of considering the dielectric property and the mechanical property is solved. The innovative process guarantees stable performance, is compatible with an existing production line, can realize large-scale production, meets the requirements of fields such as new energy and the like, and provides a feasible scheme for localization of high-performance capacitor films.

Description

Technical Field

[0001] This invention relates to the field of capacitor film technology, and in particular to a preparation process for a multilayer polypropylene capacitor film. Background Technology

[0002] Capacitor films are a key basic material for electronic components, widely used in capacitors, transformers, and other equipment. Their dielectric properties, mechanical strength, and environmental adaptability directly determine the operating efficiency and lifespan of electronic devices. With the electronics industry developing towards higher frequencies, smaller sizes, and higher power, especially with the rapid expansion of new energy vehicles, photovoltaic energy storage, and 5G communications, more stringent requirements are being placed on the performance of capacitor films. They not only need to possess high dielectric constants to increase energy storage density, but also excellent breakdown strength, anti-aging capabilities, and moisture-proof properties to adapt to long-term stable operation under complex working conditions.

[0003] Currently, mainstream capacitor film materials are mainly polymers such as polypropylene and polyethylene naphthalate, but their performance has significant shortcomings. While single polypropylene films offer good insulation, they have a low dielectric constant and insufficient temperature resistance, making them prone to deformation at high temperatures. Polyethylene naphthalate films, although possessing superior dielectric properties, suffer from poor mechanical strength and weak tear resistance, making them unsuitable for high-voltage applications. To improve performance, existing technologies often employ modification by adding nanoparticles, such as mixing barium titanate into polypropylene to increase the dielectric constant. However, nanoparticles are prone to agglomeration, leading to internal defects and reducing breakdown strength. Some solutions attempt to optimize performance through multilayer composite structures, but the interlayer bonding process is often simple, resulting in weak adhesion and delamination and slippage during long-term use, severely impacting equipment reliability.

[0004] Furthermore, existing manufacturing processes also have significant limitations. Traditional capacitor film production often employs single-layer extrusion or simple composite processes, lacking control over the interfaces between different material layers, resulting in poor performance uniformity. Cooling and curing processes typically use a single temperature parameter, easily causing stress concentration within the film, leading to cracking or warping. Moisture-proof coatings are often applied in a single-step curing process, resulting in insufficient coating density and high water vapor permeability, which can easily lead to insulation failure of capacitor components in humid environments. Simultaneously, as electronic devices increasingly demand thinner and lighter materials, existing processes struggle to maintain performance while reducing film thickness, hindering the miniaturization of equipment. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention discloses a fabrication process for a multilayer polypropylene capacitor film that can meet the performance requirements of high-end electronic devices and synergistically optimize material structure and fabrication process.

[0006] This invention discloses a preparation process for a multilayer polypropylene capacitor film, comprising the following steps: S1: Raw material preparation, in which base resin, nano-ceramic filler, nucleating agent and antioxidant are respectively formulated into multilayer raw materials; S2: Multilayer co-extrusion molding, in which the multilayer raw materials are melted separately by an independent screw extruder and then compounded by a co-extrusion die to form a composite film preform; S3: Gradient cooling and shaping, in which the composite film preform is subjected to gradient cooling through an air ring; S4: Biaxial stretching treatment, in which the cooled film preform is subjected to longitudinal stretching and transverse stretching; S5: Heat setting and surface treatment, in which heat setting is performed by a segmented heating furnace, followed by corona treatment to improve surface energy; S6: Nano-coating curing, in which the outer coating is cured in segments by an infrared oven.

[0007] Further, in step S1, a three-layer raw material is prepared. The middle layer is a polypropylene core insulation layer with embedded nano-oxide coated fibers. The outer two layers are a polypropylene substrate layer doped with nano-alumina particles and a polypropylene protective layer with a nano-silica moisture-proof coating on the surface. The nano-alumina particles have a particle size of 50-200 nm and are added at 3%-8% of the mass of the polypropylene substrate layer. The nano-silica moisture-proof coating has a thickness of 0.5-2 μm and is composed of silica and alumina in a mass ratio of 2-3:1. The nano-oxide coated fibers have a diameter of 50-200 nm, a coating thickness of 5-20 nm, and a fiber content of 1%-3% of the mass of the polypropylene protective layer.

[0008] Further, in step S1, a five-layer raw material is prepared; the first layer is a polypropylene substrate layer, doped with nano-alumina particles; the second layer is a polyethylene naphthalate functional layer, doped with nano-barium titanate particles; the third layer is a polypropylene core insulating layer, embedded with nano-oxide coated fibers; the fourth layer is a polyvinylidene fluoride functional layer, doped with nano-silica particles; and the fifth layer is a polypropylene protective layer, with a nano-silica moisture-proof coating on its surface; in S1, the nano-alumina particles have a particle size of 50-200 nm and are added at 3%-8% of the mass of the first layer; the nano-barium titanate particles have a particle size of 20-100 nm and are added at 5%-10% of the mass of the second layer. The nano-silica has a particle size of 30-150nm and is added at 4%-9% of the mass of the fourth layer. The nano-silica moisture-proof coating in the fifth layer has a thickness of 0.5-2μm and is composed of silica and alumina in a mass ratio of 2-3:1. The thickness ratio of the five layers is 1-3:2-4:3-5:2-4:1-3, and the total film thickness is 50-100μm. The surface of the nano-ceramic filler is treated with a silane coupling agent to enhance the interfacial bonding force with the polymer matrix. The nano-oxide coating fibers in the third layer have a diameter of 50-200nm, a coating thickness of 5-20nm, and a fiber content of 1%-3% of the mass of the third layer.

[0009] Furthermore, the preparation method of the third layer of nano-oxide coated fiber in S1 includes preparing PAN or PET nanofibers by electrospinning; coating the fiber surface with a nano-oxide layer by atomic layer deposition or sol-gel method; dispersing the coated fiber in a polypropylene matrix and forming the third layer by blending extrusion; the fifth layer of nano-silica moisture-proof coating in S1 is coated by comma roller coating, and then calendered by hot press roller; the pressure of the hot press roller is 0.5-1.2 MPa, and the roller surface temperature is 140-160℃.

[0010] Furthermore, in S1, the raw materials for each layer also include: Layer 1: 81-95.5 parts of polypropylene resin, 3-8 parts of nano-alumina particles, 1-3 parts of nucleating agent, and 0.5-2 parts of antioxidant; Layer 2: 83.5-93 parts of polyethylene naphthalate, 5-10 parts of nano-barium titanate particles, 1-2 parts of nucleating agent, and 0.5-1.5 parts of antioxidant; Layer 3: 93-96 parts of polypropylene resin, 1-3 parts of nano-oxide coated fiber, 2-4 parts of nucleating agent, and 1-3 parts of antioxidant; Layer 4: 80-91 parts of polyvinylidene fluoride, 4-9 parts of nano-silica particles, 1-3 parts of nucleating agent, and 0.5-2 parts of antioxidant; Layer 5: 85.5-98 parts of polypropylene resin, 1-2 parts of nucleating agent, and 0.5-1.5 parts of antioxidant, and the surface is coated with a nano-silica moisture-proof coating with a thickness of 0.5-2μm.

[0011] Furthermore, the nucleating agent is at least one of sorbitol, phosphate esters, or metal salts of organic carboxylic acids; the antioxidant is at least one of hindered phenols, phosphites, or thioesters.

[0012] Furthermore, nano-coated fibers are added to the contact surfaces of the first and second layers, and the fourth and fifth layers, respectively. Specifically, PET nanofibers are dispersed on the side of the first layer closest to the second layer, and the surface of the fibers is coated with a nano-alumina coating by atomic layer deposition. The diameter of the PET nanofibers is 50-150 nm, the coating thickness is 5-15 nm, and the fiber content accounts for 0.5%-1.5% of the mass of the first layer. On the side of the fourth layer closest to the fifth layer, PVDF nanofibers are dispersed, and the surface of the fibers is coated with a nano-silica coating by sol-gel method. The diameter of the PVDF nanofibers is 80-200 nm, the coating thickness is 5-20 nm, and the fiber content accounts for 0.5%-1.5% of the mass of the fourth layer. The nano-coated fibers are added as follows: during the raw material blending stage of the first and fourth layers, the pretreated nano-coated fibers are mixed with the base resin, nano-ceramic filler, nucleating agent, and antioxidant, and then dispersed evenly by a screw extruder.

[0013] Furthermore, in S2, the die temperature of the independent screw extruder is controlled at 200-240℃; in the multi-layer co-extrusion molding in S2, the extrusion temperatures of each layer of raw material are as follows: Layer 1 and Layer 5: 160-180℃; Layer 2, Layer 3, and Layer 4: 180-220℃; in S2, when extruding the composite film preform, the film nozzle diameter is ≥1.8cm, and the initial clamping roller pressure is set to 20%-30% of the maximum setting value; during the film stretching process, the clamping roller pressure is gradually increased to the maximum setting value by an increment of 5%-10% each time, and the maximum setting value is 1.5-3MPa.

[0014] Furthermore, the inner airflow of the S3 wind ring is 150-200 m³ / h. 3 / h, external air circulation volume is 80-120m³ 3 / h, the inner air ring temperature is controlled at 15-25℃, the outer air ring temperature is controlled at 25-35℃, and the cooling line height is maintained at 1.2-1.8m; in the S4 biaxial stretching process, the longitudinal stretching ratio is 3-5 times, the longitudinal stretching temperature is 120-130℃, the transverse stretching ratio is 4-6 times, and the transverse stretching temperature is 130-150℃; during the S4 stretching process, directional indentation lines are set at the edge of the preform and in areas requiring directional stretching, so that pre-formation is formed at the indentation lines. The orientation structure and directional indentation lines are formed by laser etching or mechanical imprinting, with a line width of 0.1-0.3 mm and a spacing of 5-10 mm. In the S5 segmented heating furnace, the temperature of the crystallization zone is controlled at 100-140℃, and the temperature of the setting zone is controlled at 150-170℃. During the S5 heat setting process, the surface roughness Ra of the hot press roller is ≤0.1μm, the roller surface temperature is controlled by heat transfer oil circulation, the power of the corona treatment is 1-3kW, and the processing speed is 50-100m / min.

[0015] Further, step S6, specifically the segmented curing of the outer coating using an infrared oven, involves: segmented curing of the 5th layer of silica moisture-proof coating using an infrared oven; the specific parameters for curing the nano-coating in S6 are as follows: three-stage segmented curing using an infrared oven; the first stage temperature is 70-90℃, curing time 10-15 min; the second stage temperature is 110-130℃, curing time 8-12 min; the third stage temperature is 140-160℃, curing time 5-8 min; the hot air circulation rate of the infrared oven is 2-3 m / s, and the water vapor permeability of the cured coating is ≤1.0 g / (m²). 2 •24h).

[0016] The beneficial effects of this invention are:

[0017] This invention achieves a breakthrough improvement in the overall performance of capacitor films through multi-layer heterogeneous structure design and material matching. The polypropylene substrate layer and protective layer provide mechanical support, while the polyethylene naphthalate and polyvinylidene fluoride functional layers significantly enhance the dielectric constant through high-dielectric nanoparticles. The nano-oxide coating fibers of the core insulating layer form a three-dimensional reinforcing network, increasing the breakdown strength to 45-50 kV / mm, thus solving the problem of traditional capacitor films struggling to balance dielectric performance and mechanical strength. Simultaneously, the outer nano-silica moisture-proof coating and the interlayer nanofiber reinforcement design reduce water vapor permeability to 0.5-0.8 g / (m²). 2 • 24h), the interlayer peel strength reaches 3.2-5.5N / cm, effectively resisting the erosion of humid environments and avoiding interlayer slippage, greatly extending the service life of capacitor components.

[0018] During multi-layer co-extrusion, independent temperature control and gradient pressure adjustment ensure sufficient melting of each layer and tight interfacial bonding. Gradient cooling and biaxial stretching work synergistically to reduce intrafilm stress and promote molecular chain orientation, increasing tensile strength to 180-200 MPa. Segmented heat setting and three-stage curing processes further refine the crystal structure and densify the coating, avoiding defects such as cracking and blistering caused by rapid cooling or curing in traditional processes. The control of these process parameters keeps the product performance deviation rate within 5%, far lower than the fluctuation range of over 15% in existing technologies, ensuring the stability of mass production.

[0019] This invention demonstrates significant adaptability and industrial value in practical applications. Compared to products with single-layer structures or simplified processes, its dielectric constant is increased by more than 50%, its breakdown strength is improved by 40%-60%, and its moisture resistance is optimized by more than 60%, meeting the demands of new energy vehicles, energy storage devices, and other applications for high-reliability capacitor components. Furthermore, the process route is compatible with existing multi-layer co-extrusion production lines; large-scale production can be achieved by adjusting screw parameters and temperature control programs without large-scale equipment modifications. Combining technological innovation and industrial feasibility, this invention provides a practical solution for the domestic production of high-performance capacitor films. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the preparation process of a multilayer polypropylene capacitor film according to an embodiment of this application. Detailed Implementation

[0021] To enable those skilled in the art to better understand the present invention, the technical solutions in the specific embodiments of the present invention will be clearly and completely described below.

[0022] This invention discloses a process for preparing a multilayer polypropylene capacitor film, which includes the following steps:

[0023] S1: Raw material preparation. The base resin, nano-ceramic filler, nucleating agent, and antioxidant are formulated into multi-layer raw materials. The first layer is a polypropylene substrate layer doped with nano-alumina particles; the second layer is a polyethylene naphthalate functional layer doped with nano-barium titanate particles; the third layer is a polypropylene core insulating layer with embedded nano-oxide coated fibers; the fourth layer is a polyvinylidene fluoride functional layer doped with nano-silica particles; and the fifth layer is a polypropylene protective layer with a nano-silica moisture-proof coating on the surface.

[0024] S2: Multi-layer co-extrusion molding, in which multiple layers of raw materials are melted separately by an independent screw extruder and then combined through a co-extrusion die to form a composite preform;

[0025] S3: Gradient cooling and shaping, the composite film preform is cooled by gradient cooling through an air ring;

[0026] S4: Biaxial stretching treatment, which involves longitudinal and transverse stretching of the cooled preform;

[0027] S5: Heat setting and surface treatment, heat setting is carried out in a segmented heating furnace, followed by corona treatment to improve surface energy;

[0028] S6: Nano-coating curing, the fifth layer of silica moisture-proof coating is cured in stages using an infrared oven.

[0029] S1: Raw material preparation achieves material performance matching through layered design. The first layer uses a polypropylene substrate doped with nano-alumina particles, utilizing the mechanical stability of polypropylene and the thermal conductivity of alumina to provide basic structural support for the capacitor film. The second layer uses polyethylene naphthalate doped with nano-barium titanate, leveraging the high dielectric constant of barium titanate to improve the film's capacitance performance. The third layer uses polypropylene as the matrix with embedded nano-oxide coating fibers; the fiber-reinforced structure improves the breakdown strength of the insulation layer. The fourth layer uses polyvinylidene fluoride doped with nano-silica, utilizing its chemical resistance and the high-temperature resistance of silica to optimize the film's environmental adaptability. The fifth layer has a polypropylene surface coated with a nano-silica moisture-proof coating, forming a barrier to prevent moisture intrusion. Layered raw material design can integrate the advantages of different materials to meet the comprehensive requirements of capacitor films for mechanical strength, dielectric properties, insulation, and moisture resistance.

[0030] S2: Multi-layer co-extrusion molding uses an independent screw extruder to melt multiple layers of raw materials separately, allowing control over the melting state of each layer and avoiding mixing interference caused by differences in melting temperature between different materials. The composite process of the co-extrusion die enables one-time molding of multi-layer structures, reducing process losses during interlayer bonding and ensuring tight bonding between each layer. This improves production efficiency and ensures uniform interlayer thickness of the composite preform by controlling extrusion parameters, providing a stable basic shape for subsequent processing.

[0031] S3: Gradient cooling for shaping. The gradient cooling design of the air ring allows for staged control of the cooling rate of the membrane preform. The inner and outer air rings form a temperature gradient, preventing deformation or cracking of the membrane preform due to internal stress caused by rapid cooling. Gradient cooling allows the membrane preform to gradually shape during slow temperature changes, maintaining the structural stability of each layer, reducing the risk of delamination between layers due to different shrinkage rates, and improving the dimensional accuracy of the membrane.

[0032] S4: Biaxial stretching treatment combines longitudinal and transverse stretching, enabling the molecular chains within the membrane to align in an orderly manner along the stretching direction, thus improving the membrane's mechanical strength and tensile properties. Biaxial stretching also makes the crystal structure of each layer more regular, reducing internal defects and thereby enhancing the membrane's insulation performance and dielectric consistency. This treatment comprehensively improves the membrane's overall mechanical properties, meeting the mechanical requirements of capacitor components during assembly and use.

[0033] S5: Heat setting and surface treatment. The segmented heating furnace can eliminate residual stress in the film. By controlling different temperature ranges, the dimensional stability of the film can be further improved, avoiding shrinkage due to temperature changes during subsequent use. Corona treatment can increase the polar groups on the film surface, improve the surface energy, and provide good interfacial bonding for possible subsequent electrode bonding or other treatments, ensuring the assembly reliability of capacitor components.

[0034] S6: The segmented curing of the nano-coating infrared oven heats the 5th layer of silica moisture-proof coating in stages, allowing the solvent in the coating to gradually evaporate, avoiding cracking or bubbling caused by rapid curing; the cured coating forms a dense structure, effectively blocking moisture from entering the membrane and extending the service life of the capacitor membrane, especially suitable for capacitor components used in humid environments.

[0035] In one implementation method, the nano-alumina particles in layer S1 have a particle size of 50-200 nm and are added at 3%-8% of the mass of the first layer. The nano-barium titanate particles have a particle size of 20-100 nm and are added at 5%-10% of the mass of the second layer. The nano-silica particles have a particle size of 30-150 nm and are added at 4%-9% of the mass of the fourth layer. The nano-silica moisture-proof coating in layer 5 has a thickness of 0.5-2 μm and is composed of silica and alumina in a mass ratio of 2-3:1. The thickness ratio of the five layers is 1-3:2-4:3-5:2-4:1-3, and the total film thickness is 50-100 μm. The surface of the nano-ceramic filler is treated with a silane coupling agent to enhance the interfacial bonding with the polymer matrix. The nano-oxide coating fibers in layer 3 have a diameter of 50-200 nm, a coating thickness of 5-20 nm, and a fiber content of 1%-3% of the mass of the third layer.

[0036] The first layer consists of nano-alumina particles with a particle size of 50-200 nm, mixed with the polypropylene substrate at a ratio of 3%-8% by weight of the first layer, and dispersed uniformly by high-speed stirring or melt blending. This particle size range balances particle dispersibility and mechanical reinforcement, avoiding agglomeration caused by excessively fine particles or interface defects caused by excessively coarse particles. The 3%-8% addition amount can utilize the high thermal conductivity of alumina to improve heat dissipation capacity without affecting the melt flowability of polypropylene due to excessive content, ensuring stable interlayer bonding.

[0037] The second layer uses 20-100nm barium titanate nanoparticles, incorporated into polyethylene naphthalate at a ratio of 5%-10% of the second layer's mass. After ultrasonic dispersion pretreatment, the mixture is melt-composite. The small particle size of 20-100nm increases the specific surface area of ​​the particles, enhances the interfacial interaction with the matrix, and strengthens the dielectric properties. The 5%-10% addition fully utilizes the high dielectric constant of barium titanate while avoiding excessive addition that could increase material brittleness, ensuring the flexibility of the functional layer.

[0038] The fourth layer consists of 30-150nm nano-silica particles, mixed with polyvinylidene fluoride (PVDF) at a ratio of 4%-9% by weight of the fourth layer, and uniformly dispersed by the shearing action of a screw extruder. The 30-150nm particle size matches the molecular chain spacing of PVDF, enhancing interfacial compatibility; the 4%-9% addition utilizes the high-temperature resistance of silica to improve interlayer anti-aging ability, while its rigidity improves the dimensional stability of PVDF without compromising its chemical corrosion resistance.

[0039] The fifth layer is prepared by mixing silica and alumina in a mass ratio of 2-3:1 to form a coating liquid. This liquid is then applied to the surface of the fifth layer of polypropylene using a comma roller, with the coating thickness controlled at 0.5-2 μm. In this composite ratio, silica is the main component, forming a dense moisture barrier, while alumina helps improve the coating's wear resistance. The 0.5-2 μm thickness effectively blocks moisture without causing cracking due to excessive thickness, thus balancing moisture resistance and mechanical compatibility.

[0040] By controlling the flow rate of the co-extrusion die, the thickness ratio of the five layers is maintained at 1-3:2-4:3-5:2-4:1-3, with the total film thickness controlled at 50-100μm. The core insulating layer, layer 3, has the highest proportion (3-5%) to ensure breakdown strength; the functional layers, layers 2 and 4, have a moderate proportion (2-4%) to balance dielectric performance and material cost; the protective and substrate layers, layers 1 and 5, have a reasonable proportion (1-3%) to provide structural support. The total film thickness of 50-100μm meets the requirements for thinner and lighter capacitor components while ensuring mechanical strength. The nano-ceramic filler is impregnated or sprayed with a silane coupling agent to form a molecular-level interface modification layer. The silane coupling agent can chemically bond ceramic particles to the polymer matrix, eliminating interfacial voids, reducing the risk of interlayer delamination, and improving the overall mechanical strength and impact resistance of the material.

[0041] The third layer consists of nanofibers with a diameter of 50-200 nm, such as PAN or PET. A 5-20 nm thick nanofiber coating is applied to the surface of these fibers via atomic layer deposition or a sol-gel method, and dispersed in polypropylene at a ratio of 1%-3% by weight of the third layer. The fine-diameter fibers (50-200 nm) can be uniformly distributed in the insulation layer, forming a three-dimensional reinforcing network; the thin coating (5-20 nm) maximizes the interfacial interaction between the fibers and the matrix, improving breakdown strength; and the 1%-3% content can improve the tensile and tear resistance of the core layer without affecting insulation performance.

[0042] In one embodiment, the preparation method of the third layer of nano-oxide coated fiber in S1 includes preparing PAN or PET nanofibers by electrospinning. A nano-oxide layer is then coated onto the fiber surface using atomic layer deposition or a sol-gel method. The coated fibers are dispersed in a polypropylene matrix and the third layer is formed by co-extrusion. The fifth layer of nano-silica moisture-proof coating in S1 is coated using a comma roller coating method, followed by calendering using a hot press roller. The pressure of the hot press roller is 0.5-1.2 MPa, and the roller surface temperature is 140-160°C.

[0043] PAN or PET nanofibers are prepared by electrospinning. The PAN or PET polymer is dissolved in a suitable solvent to prepare a spinning solution. A high-voltage electrostatic field is used to form a Taylor cone at the spinning nozzle. After solvent evaporation, a nanofiber film with uniform diameter is obtained, which is then cured to obtain PAN or PET nanofibers. These nanofibers are placed in an atomic layer deposition (ALD) apparatus, and a nano-oxide film is grown layer by layer on the fiber surface by alternately introducing precursor gases such as trimethylaluminum alumina and water. Alternatively, a sol-gel method is used, in which the fibers are immersed in an oxide sol such as silica sol, pulled dry, and then calcined at high temperature to form a nano-oxide coating, with the coating thickness controlled at 5-20 nm. The coated nano-oxide fibers are cut into short fiber segments and mixed with polypropylene resin, nucleating agent, and antioxidant in a specific ratio. The mixture is then sheared and dispersed in a molten state using a twin-screw extruder, ensuring uniform fiber distribution within the polypropylene matrix. The resulting material forms the third layer of raw material. Electrospinning can prepare nanofibers with uniform diameters of 50-200 nm and high aspect ratios, providing a continuous reinforcing skeleton for the insulation layer and improving mechanical properties. Atomic layer deposition or sol-gel methods can achieve uniform coating of oxides with controllable thicknesses of 5-20 nm, allowing for regulation of the interfacial bonding between the fibers and the polypropylene matrix and preventing fiber agglomeration. Blending extrusion processes ensure uniform dispersion of coated fibers in polypropylene, forming a three-dimensional network structure, improving the puncture resistance and tear resistance of the third layer while maintaining good insulation.

[0044] A coating solution prepared by mixing nano-silica and alumina in a specific ratio, with the solid content adjusted to a suitable viscosity for coating, is used. The coating amount is controlled by the gap between the comma-shaped doctor blade and the substrate, forming a continuous wet coating on the fifth layer of polypropylene, ensuring a coating thickness of 0.5-2 μm. Before the coating is fully cured, a hot press roller is introduced to calender the coating surface, setting the roller surface pressure to 0.5-1.2 MPa and the roller surface temperature to 140-160℃. Hot pressing densifies the coating and ensures a tight bond with the substrate. The comma-shaped roller coating offers high precision and can stably control the coating thickness from 0.5-2 μm, avoiding uneven moisture-proof performance caused by localized over-thickness or under-thickness, making it suitable for large-scale continuous production. The 0.5-1.2 MPa pressure of the hot press roller eliminates air bubbles within the coating, allowing nanoparticles to pack tightly and form a dense moisture barrier; the 140-160℃ temperature promotes initial curing of the coating without causing thermal deformation of the polypropylene substrate, while simultaneously improving the interfacial adhesion between the coating and the fifth layer. Calendering can reduce the surface roughness of the coating, reduce the water vapor penetration path, and, in conjunction with the subsequent curing process, reduce the water vapor permeability of the final coating, thereby enhancing the long-term moisture-proof stability of the capacitor film.

[0045] In one embodiment, in S1, the raw materials for each layer further include: Layer 1: 81-95.5 parts of polypropylene resin, 3-8 parts of nano-alumina particles, 1-3 parts of nucleating agent, and 0.5-2 parts of antioxidant. Layer 2: 83.5-93 parts of polyethylene naphthalate, 5-10 parts of nano-barium titanate particles, 1-2 parts of nucleating agent, and 0.5-1.5 parts of antioxidant. Layer 3: 93-96 parts of polypropylene resin, 1-3 parts of nano-oxide coated fiber, 2-4 parts of nucleating agent, and 1-3 parts of antioxidant. Layer 4: 80-91 parts of polyvinylidene fluoride, 4-9 parts of nano-silica particles, 1-3 parts of nucleating agent, and 0.5-2 parts of antioxidant. Layer 5: 85.5-98 parts of polypropylene resin, 1-2 parts of nucleating agent, and 0.5-1.5 parts of antioxidant, with a nano-silica moisture-proof coating of 0.5-2 μm thickness on the surface.

[0046] The first layer of raw materials is prepared by mixing 81-95.5 parts of polypropylene resin with 3-8 parts of nano-alumina particles in a specific ratio. The mixture is dispersed for 10-15 minutes by high-speed stirring at 1000-1500 rpm. Then, 1-3 parts of nucleating agent and 0.5-2 parts of antioxidant are added, and stirring continues for 5-8 minutes. The mixture is then melt-blended using a twin-screw extruder at 170-190℃ and granulated to form the first layer of raw materials. The high proportion of 81-95.5 parts of polypropylene resin ensures the mechanical stability of the substrate layer. The addition of 3-8 parts of nano-alumina enhances thermal conductivity while preventing agglomeration. The nucleating agent promotes resin crystallization and refines dimensional stability. The antioxidant inhibits oxidative degradation during high-temperature processing, extending the material's service life.

[0047] The second layer of raw material preparation involves mixing 83.5-93 parts of polyethylene naphthalate resin with 5-10 parts of nano-barium titanate particles. The mixture is melted in a screw extruder at 240-260℃. 1-2 parts of a nucleating agent and 0.5-1.5 parts of an antioxidant are added, and the mixture is further dispersed using a static mixer. After extrusion, the mixture is cooled and pelletized. Polyethylene naphthalate constitutes the majority of the mixture (3.5-93 parts) to ensure the high-temperature resistance and mechanical strength of the functional layer. The 5-10 parts of nano-barium titanate improve the dielectric constant, and the amount added is controlled within a range that does not disrupt the resin's continuity. Small amounts of nucleating agent and antioxidant optimize the crystal structure and prevent thermo-oxidative aging during processing.

[0048] The third layer of raw material preparation involves taking 93-96 parts of polypropylene resin and 1-3 parts of nano-oxide coated fiber, pre-treated with surface activation, mixing, and then passing through a high-shear mixer at a shear rate of 2000-3000 s. -1 Disperse for 20-30 minutes, then add 2-4 parts nucleating agent and 1-3 parts antioxidant. Extrude the mixture using a twin-screw extruder at 180-200℃ to ensure uniform fiber distribution. A high proportion of polypropylene resin (93-96 parts) provides a stable matrix for the insulation layer. 1-3 parts of nano-oxide coating fibers form a three-dimensional reinforcing network, improving breakdown strength. A higher nucleating agent content (2-4 parts) promotes rapid resin crystallization and enhances structural stability. An increased antioxidant proportion (1-3 parts) meets the long-term use requirements of the core layer under high-temperature environments.

[0049] The fourth layer of raw material preparation involves heating 80-91 parts of polyvinylidene fluoride (PVDF) resin to 180-200℃ to melt it, adding 4-9 parts of nano-silica particles, pre-treating by ultrasonic dispersion for 30 minutes, stirring and mixing for 15-20 minutes, then adding 1-3 parts of nucleating agent and 0.5-2 parts of antioxidant. The mixture is then extruded and granulated using a screw extruder at 200-220℃. The 80-91 parts of PVDF fully utilize its chemical resistance and high-temperature resistance, while the 4-9 parts of nano-silica refine the crystal structure and improve the material's thermal stability. The appropriate addition of nucleating agent and antioxidant balances processing fluidity and long-term anti-aging performance.

[0050] The fifth layer is prepared by mixing 85.5-98 parts of polypropylene resin with 1-2 parts of nucleating agent and 0.5-1.5 parts of antioxidant. This mixture is then melt-extruded to form the substrate layer. Subsequently, a nano-silica moisture-proof coating is applied using a comma roller coating system. Silica and alumina are mixed in a specific ratio and coated onto the surface. Immediately after coating, the mixture is passed through a hot press roller at a pressure of 0.5-1.2 MPa and a temperature of 140-160℃ for calendering. The high proportion of polypropylene resin (85.5-98 parts) ensures the mechanical strength and processing stability of the protective layer. The small amount of nucleating agent and antioxidant reduces oxidation and crystallization defects in the substrate during subsequent processing. The hot press roller treatment ensures a tight bond between the moisture-proof coating and the substrate, improving coating adhesion and density, and ensuring long-lasting and stable moisture-proof performance.

[0051] In one embodiment, the nucleating agent is at least one of sorbitol, phosphate esters, or metal salts of organic carboxylic acids. The antioxidant is at least one of hindered phenols, phosphites, or thioesters.

[0052] Select appropriate nucleating agents based on the type of resin in each layer. For polypropylene layers 1, 3, and 5, sorbitol-based nucleating agents, such as dibenzyl sorbitol, can be added and dispersed evenly during melt blending with the resin. For the polyethylene naphthalate layer 2, phosphate-based nucleating agents, such as metal phosphate salts, are used, added during the resin melting stage and dispersed by stirring. For the polyvinylidene fluoride layer 4, organocarboxylic acid metal salt nucleating agents, such as calcium stearate, can be used, achieving uniform distribution through high-speed shear mixing. Different types of nucleating agents can also be compounded as needed, with the compounding ratio adjusted from 1:1 to 3:1.

[0053] Sorbitol-based nucleating agents refine the crystal structure of polypropylene, improving film transparency and impact resistance; phosphate ester-based nucleating agents are compatible with the high-temperature processing characteristics of polyethylene naphthalate, promoting crystallization rate and reducing molding shrinkage; organocarboxylic acid metal salts have good compatibility with polyvinylidene fluoride, improving its crystallinity and heat distortion resistance. When used in combination, the advantages of different nucleating agents can be combined to further optimize the mechanical properties and dimensional stability of each layer.

[0054] Antioxidants are selected based on the processing temperature and operating environment of each layer. Hindered phenolic antioxidants, such as 1010, are added to layers 1, 3, and 5 of the polypropylene layer, during the initial melt blending stage with the resin. For the high-temperature processed polyethylene naphthalate layer 2, a phosphite antioxidant, such as 168, is added simultaneously with the raw materials during the extrusion stage. For the polyvinylidene fluoride layer 4, a thioester antioxidant, such as DLTP, can be used, which is premixed and then melt-dispersed with the resin. If necessary, a combination of primary and secondary antioxidants can be used, such as a 2:1 mixture of hindered phenols and phosphites, to enhance the antioxidant effect.

[0055] Hindered phenolic antioxidants effectively capture free radicals, inhibiting the oxidative degradation of polypropylene during processing and use; phosphites, as auxiliary antioxidants, can decompose hydroperoxides, adapting to the high-temperature processing requirements of polyethylene naphthalate and reducing thermo-oxidative aging; thioester antioxidants in polyvinylidene fluoride can improve its weather resistance and prevent performance degradation during long-term use. When used in combination, they create a synergistic effect, significantly extending the service life of each layer of material, especially suitable for capacitor films operating in high-temperature environments for extended periods.

[0056] In one implementation, nano-coated fibers are added to the contact surfaces of the first and second layers, and the fourth and fifth layers. Specifically, PET nanofibers are dispersed on the side of the first layer closest to the second layer. The surface of the fibers is coated with a nano-alumina coating via atomic layer deposition. The PET nanofibers have a diameter of 50-150 nm, the coating thickness is 5-15 nm, and the fiber content accounts for 0.5%-1.5% of the mass of the first layer. PVDF nanofibers are dispersed on the side of the fourth layer closest to the fifth layer. The surface of the fibers is coated with a nano-silica coating via a sol-gel method. The PVDF nanofibers have a diameter of 80-200 nm, the coating thickness is 5-20 nm, and the fiber content accounts for 0.5%-1.5% of the mass of the fourth layer. The nano-coated fibers are added as follows: during the raw material blending stage of the first and fourth layers, the pretreated nano-coated fibers are mixed with the base resin, nano-ceramic filler, nucleating agent, and antioxidant, and then dispersed uniformly using a screw extruder.

[0057] The nano-coated fibers at the interface between the first and second layers are prepared using electrospinning with a diameter of 50-150 nm PET nanofibers. A 5-15 nm thick nano-alumina coating is then deposited onto the fiber surface via atomic layer deposition. During the first layer raw material blending stage, pretreated PET nanofibers are added at a ratio of 0.5%-1.5% of the first layer's mass, mixed with polypropylene resin, nano-alumina particles, nucleating agents, and antioxidants. The mixture is then sheared and dispersed using a twin-screw extruder at a speed of 300-500 r / min and a temperature of 170-190℃, ensuring the fibers are evenly distributed on the side of the first layer closest to the second layer.

[0058] PET nanofibers have good compatibility with the second layer of polyethylene naphthalate. The surface alumina coating can form a chemical adsorption with the first layer of polypropylene, enhancing the interlayer bonding force. The fine diameter of 50-150nm and the thin coating of 5-15nm can reduce the influence of fibers on the interlayer flatness. The low content of 0.5%-1.5% can form a three-dimensional network reinforcement structure and avoid interlayer defects caused by excessive fibers, thereby improving peel strength and fatigue resistance.

[0059] The nano-coated fibers at the interface between the fourth and fifth layers are prepared as PVDF nanofibers with a diameter of 80-200 nm through electrospinning. A 5-20 nm thick nano-silica coating is then applied to the fiber surface using a sol-gel method. During the blending stage of the fourth layer, this fiber is added at a ratio of 0.5%-1.5% of the fourth layer's mass, mixed with polyvinylidene fluoride resin, nano-silica particles, nucleating agents, and antioxidants. The mixture is then extruded using a screw extruder at a temperature of 200-220℃ and a shear rate of 1500-2000 s. -1 Melt dispersion ensures that the fibers are oriented on the side of the 4th layer closest to the 5th layer.

[0060] PVDF nanofibers exhibit excellent compatibility with the fourth layer of polyvinylidene fluoride matrix. The surface silica coating can form hydrogen bonds with the fifth layer of moisture-proof coating, strengthening the interlayer interface bonding. The fiber diameter of 80-200nm is adapted to the thickness ratio of the fourth and fifth layers, and the coating thickness of 5-20nm balances the interlayer bonding and fiber flexibility. The content of 0.5%-1.5% can improve the interlayer shear strength and resistance to damp heat aging without affecting the dielectric properties, reducing the risk of interlayer delamination during use.

[0061] In one implementation, the die temperature of the independent screw extruder in S2 is controlled at 200-240℃. In the multi-layer co-extrusion molding in S2, the extrusion temperatures of each layer of raw material are as follows: Layer 1 and Layer 5: 160-180℃; Layers 2, 3, and 4: 180-220℃. In S2, during the extrusion of the composite preform, the film nozzle diameter is ≥1.8cm, and the initial clamping roller pressure is set to 20%-30% of the maximum set value. During the film stretching process, the clamping roller pressure is gradually increased to the maximum set value in increments of 5%-10%, with the maximum set value being 1.5-3MPa.

[0062] The die head of the independent screw extruder employs a segmented temperature control system, with 3-5 heating zones for independent temperature regulation. Thermocouples in each zone monitor the temperature in real time and feed it back to the control system, automatically adjusting the heating power to stabilize the overall die head temperature at 200-240℃. This temperature range is higher than the melting temperature of each layer of raw material, ensuring good fluidity of the five layers within the die head, promoting molecular diffusion at the interlayer interfaces, and enhancing interlayer bonding. Simultaneously, it avoids resin degradation due to excessively high temperatures or poor interlayer adhesion caused by insufficient fluidity due to excessively low temperatures.

[0063] The extrusion temperature is set according to the melting characteristics of each resin layer. The screw extruder heating section temperature for the first and fifth layers of polypropylene is set in a gradient of 160-180℃, with the feeding section at 160℃, the melting section at 170℃, and the metering section at 180℃. The extruder temperatures for the second layer of polyethylene naphthalate, the third layer of polypropylene, and the fourth layer of polyvinylidene fluoride are set in a gradient of 180-220℃, with the feeding section at 180℃, the melting section at 200℃, and the metering section at 220℃. The raw material output rate is controlled by adjusting the screw speed (30-50 r / min) in conjunction with the temperature. Polypropylene can be completely melted at 160-180℃ and is not easily degraded, ensuring the processing stability of layers 1 and 5. Layers 2, 3, and 4 are heated to a higher temperature of 180-220℃ to meet the high melting temperature requirements of polyethylene naphthalate and polyvinylidene fluoride, ensuring that they are fully plasticized and avoiding internal defects in the preform due to insufficient melting. At the same time, it forms a reasonable temperature gradient with the die head, reducing the impact of temperature fluctuations on interlayer bonding.

[0064] A co-extrusion die with an orifice diameter ≥1.8cm is selected to ensure sufficient initial cross-sectional area during composite film extrusion. The clamping roller pressure is adjusted via a hydraulic control system. The initial pressure is set at 20%-30% of the maximum set value of 1.5-3MPa. During film stretching, the pressure is increased by 5%-10% every 3-5 seconds under program control until the maximum set value is reached. A larger orifice diameter ≥1.8cm provides ample thickness adjustment space for subsequent biaxial stretching, reducing thickness deviation during the stretching process. The initial low pressure prevents wrinkles caused by excessive force immediately after extrusion. The gradually increasing pressure enhances interlayer bonding during the film's cooling and setting process. The final pressure of 1.5-3MPa ensures a tight bond between the five layers, reducing interlayer gaps and improving the overall density and mechanical strength of the film.

[0065] In one implementation method, the airflow rate of the inner air ring in S3 is 150-200 m³ / h, and the airflow rate of the outer air ring is 80-120 m³ / h. The temperature of the inner air ring is controlled at 15-25℃, and the temperature of the outer air ring is controlled at 25-35℃. The height of the cooling line is maintained at 1.2-1.8 m. In the biaxial stretching process of S4, the stretching ratio of the longitudinal stretching is 3-5 times, and the longitudinal stretching temperature is 120-130℃. The stretching ratio of the transverse stretching is 4-6 times, and the transverse stretching temperature is 130-150℃. During the S4 stretching process, directional indentation lines are provided at the edge of the film blank and in areas requiring directional stretching, so that a pre-oriented structure is formed at the indentation lines. The directional indentation lines are formed by laser etching or mechanical imprinting, with a line width of 0.1-0.3 mm and a spacing of 5-10 mm.

[0066] The air ring adopts a double-layer structure with an inner and outer ring. The air volume of the inner air ring is adjusted to 150-200 m³ / h by a variable frequency fan. 3 / h, the external air circulation volume is controlled at 80-120m³ / h by an independent fan. 3 / h. The inner air ring receives cooling air at 15-25℃, and the outer air ring receives cooling air at 25-35℃. The airflow is evenly distributed across the film preform surface via guide plates at the air ring outlets. The cooling line height is maintained at 1.2-1.8m by adjusting the vertical distance between the air rings and the lower traction rollers, ensuring gradient cooling of the film preform during its ascent. The inner air ring's high-volume, low-temperature cooling rapidly cools the inner layer of the film preform, while the outer air ring's low-volume, higher-temperature cooling slowly cools the outer layer, creating a temperature gradient that reduces internal stress and prevents film preform curling or cracking. The 1.2-1.8m cooling line height provides sufficient cooling and setting time for the film preform, ensuring stable curing of the five-layer structure and improving dimensional accuracy in subsequent stretching processes.

[0067] Longitudinal stretching is achieved using two sets of stretching rollers with a speed difference, the front roller moving slower than the rear roller. The stretching ratio is adjusted to achieve a 3-5 times difference. The stretching rollers have built-in heating devices to control the temperature at 120-130℃. Transverse stretching is performed using a tenter frame, where chain clamps drive the film preform edges to expand laterally. Adjusting the chain clamp spacing achieves a 4-6 times difference. Hot air circulation within the tenter frame maintains the temperature at 130-150℃. A longitudinal stretching ratio of 3-5 times combined with a temperature of 120-130℃ allows the polypropylene molecular chains to align longitudinally, improving tensile strength. A transverse stretching ratio of 4-6 times and a high temperature of 130-150℃ promote lateral molecular chain orientation, enhancing the film's tear resistance. Biaxial stretching, combined with different temperature settings, achieves orderly molecular chain alignment while preventing film preform breakage due to excessive stretching force, and simultaneously improves the dielectric consistency of each layer.

[0068] Before stretching, directional indentation lines are created on the edges and orientation areas of the preform using a laser etching machine or a mechanical imprinting roller. The movement trajectory of the laser head or roller is controlled by a program to maintain a line width of 0.1-0.3 mm and a line spacing of 5-10 mm. The pre-oriented structure formed at the indentation lines guides the molecular chains to preferentially align along the indentation direction during stretching, reducing irregular shrinkage at the edges and lowering the risk of cracking. The line width and spacing balance the pre-orientation effect with the overall integrity of the film, avoiding mechanical damage caused by excessively deep indentations, while simultaneously improving the thickness uniformity after stretching and ensuring stable dielectric properties of the capacitor film.

[0069] In one implementation, the temperature of the crystallization zone in the segmented heating furnace in S5 is controlled at 100-140℃, and the temperature of the setting zone is controlled at 150-170℃. During the heat setting process in S5, the surface roughness Ra of the hot press roller is ≤0.1μm, the roller surface temperature is controlled by the circulation of heat transfer oil, the power of the corona treatment is 1-3kW, and the processing speed is 50-100m / min.

[0070] The segmented heating furnace consists of two independent temperature control units: a crystallization zone and a setting zone. The crystallization zone is heated by infrared heating tubes or electric heating wires, equipped with a temperature sensor for real-time monitoring, and a PID control system stabilizes the temperature at 100-140℃. The setting zone uses hot air circulation heating, with the temperature controlled at 150-170℃ by adjusting the fan power and heating tube output. The preform passes through both zones sequentially via conveyor rollers, with the residence time adjusted by the conveyor speed. The 100-140℃ temperature in the crystallization zone promotes crystal growth and perfection of resins such as polypropylene, reduces amorphous areas, and improves the mechanical strength of the film. The 150-170℃ temperature in the setting zone, higher than the crystallization temperature, eliminates internal stress generated during stretching, fixes the molecular chain orientation structure, and prevents dimensional shrinkage during subsequent use. This two-stage temperature setting balances crystallization perfection and stress relief, improving the dimensional stability of the film by more than 30%.

[0071] The surface of the hot press roller undergoes precision grinding and ultra-fine polishing, with a roughness Ra ≤ 0.1μm ensured by laser profilometer inspection. A spiral heat transfer oil channel is created inside the roller, with high-temperature heat transfer oil continuously circulating through a pump. Combined with temperature sensors and valve regulation, the roller surface temperature fluctuation is controlled within ±2℃. The film preform passes between the hot press roller and the pressure roller, with the contact pressure maintained at 0.3-0.5MPa. Low surface roughness prevents scratches on the film surface during hot pressing, ensuring a smooth and flat film surface and reducing dielectric loss. The circulating heat transfer oil temperature control ensures uniform roller surface temperature, preventing material degradation caused by localized overheating, while simultaneously promoting further regular arrangement of crystals within the film, increasing density and breakdown strength.

[0072] The corona treatment device employs a high-frequency, high-voltage generator, adjusting the output power to 1-3kW while controlling the distance between the electrode and the film surface at 0.5-1mm. The film preform passes through the treatment area at a speed synchronized with the production line, set at 50-100m / min. During treatment, a spark detector monitors the discharge state, automatically adjusting the power to stabilize the treatment effect. The 1-3kW power and 50-100m / min speed generate appropriate plasma etching on the film surface, introducing polar groups such as hydroxyl and carboxyl groups, increasing the surface energy to 38-45mN / m, and enhancing the adhesion of subsequent coatings or electrodes. Appropriate treatment intensity avoids performance degradation caused by excessive oxidation of the film surface, while ensuring that the treatment efficiency matches the production line speed, meeting the needs of continuous production.

[0073] As one implementation method, the specific parameters for curing the nano-coating in S6 are as follows: Curing is performed in three stages using an infrared oven. The first stage temperature is 70-90℃, and the curing time is 10-15 min. The second stage temperature is 110-130℃, and the curing time is 8-12 min. The third stage temperature is 140-160℃, and the curing time is 5-8 min. The hot air circulation rate of the infrared oven is 2-3 m / s, and the water vapor permeability of the cured coating is ≤1.0 g / (m²·24h).

[0074] The infrared oven consists of three independent temperature-controlled chambers, each equipped with an infrared heating element and a temperature sensor. The first chamber maintains a stable temperature of 70-90℃ via a temperature control system, where the film material enters via a conveyor belt and remains for 10-15 minutes. It then enters the second chamber, where the temperature is adjusted to 110-130℃ and remains for 8-12 minutes. Finally, it enters the third chamber, where the temperature rises to 140-160℃ and remains for 5-8 minutes. Each chamber is equipped with a variable frequency fan, which guides the hot air circulation through baffles. A wind speed sensor monitors and adjusts the fan power in real time, maintaining a circulation rate of 2-3 m / s. The conveyor belt speed is calculated based on the chamber length and set time to ensure accurate curing times at each stage.

[0075] The first stage, low-temperature, long-duration curing, slowly evaporates residual solvents in the coating, preventing bubbles or pinholes caused by rapid solvent evaporation and laying the foundation for coating densification. The second stage, medium-temperature treatment, promotes initial cross-linking between nanoparticles, enhancing coating structural stability while reducing thermal stress accumulation. The third stage, high-temperature, short-time treatment, completes the final curing of the coating, causing silica and alumina particles to bond tightly together to form a continuous barrier, improving coating density. A hot air circulation rate of 2-3 m / s ensures uniform temperature within the oven, preventing localized overheating that could lead to coating cracking or uneven performance. The final coating water vapor transmission rate is ≤1.0 g / (m²). 2 • 24h) can effectively prevent moisture from penetrating the inside of the capacitor film, improve its service life in humid environments, and enhance the adhesion between the coating and the fifth substrate, reducing the risk of coating peeling off during use.

[0076] The following are the test methods for each performance index in Examples 1-10 and Comparative Examples 1-5, all of which are based on existing international or industry standards and adjusted according to specific test conditions:

[0077] Dielectric constant test method: ASTM D150-22, "Standard test method for AC loss characteristics and permittivity (dielectric constant) of solid electrical insulating materials". Test conditions: ambient temperature and humidity (23±2℃ / 50±5%RH), standard atmospheric pressure (101.3kPa), frequency adjusted according to material properties.

[0078] Breakdown strength test method: ASTM D149-19, "Standard test method for dielectric breakdown voltage and dielectric strength of solid electrical insulating materials at commercial power frequencies". Test conditions: ambient temperature and humidity (23±2℃ / 50±5% RH), standard atmospheric pressure, electrode spacing set according to sample thickness.

[0079] Tensile strength test method: ISO 527-3:2020 "Plastics—Determination of tensile properties of films and sheets". Test conditions: ambient temperature and humidity (23±2℃ / 50±5% RH), tensile rate 500 mm / min.

[0080] Interlayer peel strength test method: ASTM D903-98, "Standard test method for peel or stripping properties of adhesive bonding components". Test conditions: ambient temperature and humidity (23±2℃ / 50±5% RH), peel rate of 300mm / min, and peel angle of 180°.

[0081] Water vapor transmission rate test method: ISO 15106-2-2003 "Plastics—Films and sheets—Determination of water vapor transmission rate—Part 2: Infrared detection sensor method". Test conditions: 38℃ / 100%RH.

[0082] Example 1

[0083] S1: Raw material preparation (weighing by weight)

[0084] Layer 1: Weigh 95.5 parts by weight of polypropylene resin, 3 parts by weight of nano alumina particles with a particle size of 50nm, treat with silane coupling agent KH550, 1 part by weight of sorbitol nucleating agent, and 0.5 parts by weight of hindered phenolic antioxidant. Stir at high speed for 1000r / min, melt blend after 10min, and heat at 170℃.

[0085] Layer 2: Weigh out 93 parts by weight of polyethylene naphthalate, 5 parts by weight of barium titanate nanoparticles with a particle size of 20 nm, treat with silane coupling agent KH570, 1 part by weight of phosphate nucleating agent, and 0.5 parts by weight of phosphite antioxidant, disperse ultrasonically for 30 min and then melt at 240℃.

[0086] Layer 3: Weigh out 96 parts by weight of polypropylene resin, 1 part by weight of nano-oxide coated fiber (PAN fiber diameter 50nm), 5nm by atomic layer deposition of Al2O3 coating, treat with silane coupling agent, 2 parts by weight of sorbitol nucleating agent, 1 part by weight of hindered phenolic antioxidant, mix under high shear for 2000s. -1 Extruded after 20 minutes at 180℃.

[0087] Layer 4: Weigh out 91 parts by weight of polyvinylidene fluoride, 4 parts by weight of nano-silica particles with a particle size of 30 nm, treat with silane coupling agent KH560, 1 part by weight of organocarboxylic acid metal salt nucleating agent, and 0.5 parts by weight of thioester antioxidant, and then shear and disperse after melting at 200℃.

[0088] Layer 5: Weigh 98 parts by weight of polypropylene resin, 1 part of sorbitol nucleating agent, and 0.5 parts of hindered phenolic antioxidant. After melt extrusion, coat a nano-silica moisture-proof coating with a thickness of 0.5 μm using a comma roller. The ratio of silica to alumina is 2:1. After coating, calender the coating with a hot press roller at a pressure of 0.5 MPa and a temperature of 140℃.

[0089] The five layers have a thickness ratio of 1:2:3:2:1, and the total film thickness is 50μm.

[0090] S2: Multi-layer co-extrusion molding

[0091] The die temperature of the independent screw extruder is 200℃; the extrusion temperature of the 1st / 5th layer is 160℃, and the extrusion temperature of the 2nd / 3rd / 4th layers is 180℃; the die diameter is 1.8cm; the initial clamping roller pressure is 0.3MPa, the maximum is 1.5MPa, and the pressure is increased by 5% every 3 seconds to the maximum value during film stretching.

[0092] S3: Gradient Cooling Shaping

[0093] Internal air volume 150m³ 3 / h, 15℃, outer wind ring 80m 3 / h, 25℃; cooling line height 1.2m.

[0094] S4: Biaxial stretching treatment

[0095] Longitudinal stretching 3 times, 120℃; transverse stretching 4 times, 130℃; directional indentation lines, laser etching, 0.1mm wide, 5mm spacing.

[0096] S5: Heat setting and surface treatment

[0097] Segmented heating furnace: crystallization zone 100℃, shaping zone 150℃; hot press roller surface roughness Ra=0.1μm, heat transfer oil temperature control; corona treatment power 1kW, 50m / min.

[0098] S6: Nanocoating Curing

[0099] Infrared oven three-stage curing: 70℃ / 10min-110℃ / 8min-140℃ / 5min; hot air circulation rate 2m / s; coating water vapor permeability 0.8g / (m 2 •24h).

[0100] Performance data: Dielectric constant 8.2, breakdown strength 45kV / mm, tensile strength 180MPa, interlayer peel strength 3.2N / cm.

[0101] Example 2 (weighed by weight)

[0102] S1: Raw Material Preparation

[0103] Layer 1: 88 parts polypropylene resin, 5 parts nano alumina particles with a particle size of 100 nm, treated with silane coupling agent KH550, 2 parts sorbitol nucleating agent, 1 part hindered phenolic antioxidant, melt blended at 175℃.

[0104] Second layer: 88 parts of polyethylene naphthalate, 7 parts of barium titanate nanoparticles with a particle size of 50 nm, treated with silane coupling agent KH570, 1.5 parts of phosphate nucleating agent, 1 part of phosphite antioxidant, melted at 250℃.

[0105] Layer 3: 94.5 parts polypropylene resin, 2 parts nano-oxide coated fiber, 100nm diameter PET fiber, 10nm sol-gel SiO2 coating, silane coupling agent treatment, 3 parts sorbitol nucleating agent, 1.5 parts hindered phenolic antioxidant, high-shear mixing, 2500s -1 , 25 minutes.

[0106] 4th layer: 87 parts polyvinylidene fluoride, 6 parts nano silica particles with a particle size of 80 nm, treated with silane coupling agent KH560, 2 parts organic carboxylic acid metal salt nucleating agent, 1 part thioester antioxidant, melted at 210℃.

[0107] Layer 5: 92 parts polypropylene resin, 1.5 parts sorbitol nucleating agent, 1 part hindered phenolic antioxidant, apply a moisture-proof coating with a thickness of 1μm, silica:alumina = 2.5:1, calender with hot roller, pressure 0.8MPa, temperature 150℃.

[0108] The five layers have a thickness ratio of 2:3:4:3:2, and the total film thickness is 70μm.

[0109] S2: Die head temperature 220℃, 170℃ for the first / 5th layer extrusion, 200℃ for the second / third / fourth layers; die opening 2.0cm, initial clamping roller 0.5MPa, maximum 2MPa, 8% lifting every 4 seconds.

[0110] S3: Inner wind ring 180m 3 / h, 20℃, outer wind ring 100m 3 / h, 30℃, cooling line 1.5m.

[0111] S4: Vertical 4x, 125℃, Horizontal 5x, 140℃; Indentation line, mechanical embossing, 0.2mm×7mm.

[0112] S5: Crystallization zone 120℃, shaping zone 160℃; hot press roller Ra=0.08μm; corona discharge 2kW, 70m / min.

[0113] S6: 80℃ / 12min-120℃ / 10min-150℃ / 6min, hot air 2.5m / s, water vapor transmission rate 0.6g / (m2 •24h).

[0114] Performance data: Dielectric constant 8.8, breakdown strength 48kV / mm, tensile strength 190MPa, interlayer peel strength 3.8N / cm.

[0115] Example 3 (weighed by weight)

[0116] S1: Raw Material Preparation

[0117] Layer 1: 81 parts polypropylene resin, 8 parts nano alumina (200nm), KH550 treated, 3 parts sorbitol nucleating agent, 2 parts hindered phenolic antioxidant, melted at 180℃.

[0118] Second layer: 83.5 parts of polyethylene naphthalate, 10 parts of barium titanate, 100nm, KH570 treatment, 2 parts of phosphate nucleating agent, 1.5 parts of phosphite antioxidant, melt at 260℃.

[0119] Layer 3: 93 parts polypropylene, 3 parts oxide-coated fiber, 200nm PAN fiber, 20nm ZrO2 coating, 4 parts sorbitol nucleating agent, 3 parts hindered phenolic antioxidant, shearing 3000s -1 .

[0120] 4th layer: 80 parts polyvinylidene fluoride, 9 parts silicon dioxide, 150nm, KH560 treatment, 3 parts organic carboxylic acid nucleating agent, 2 parts thioester antioxidant, melt at 220℃.

[0121] 5th layer: 85.5 parts polypropylene, 2 parts sorbitol nucleating agent, 1.5 parts hindered phenolic antioxidant, coating thickness 2μm, 3:1, hot pressing 1.2MPa / 160℃.

[0122] The five layers have a thickness ratio of 3:4:5:4:3, and a total film thickness of 100μm.

[0123] S2: Die head 240℃, 180℃ for the 1st / 5th layer, 220℃ for the 2nd / 3rd / 4th layers; Die opening 2.2cm, initial pressure 0.6MPa, maximum pressure 3MPa, 10% increase every 5 seconds.

[0124] S3: Inner wind ring 200m 3 / h, 25℃, outer wind ring 120m 3 / h, 35℃, cooling line 1.8m.

[0125] S4: Longitudinal 5 times, 130℃; Lateral 6 times, 150℃; Indentation line 0.3mm×10mm.

[0126] S5: Crystallization zone 140℃, shaping zone 170℃; hot press roller Ra=0.05μm; corona discharge 3kW, 100m / min.

[0127] S6: 90℃ / 15min-130℃ / 12min-160℃ / 8min, hot air 3m / s, water vapor transmission rate 0.5g / (m 2 •24h).

[0128] Performance data: Dielectric constant 9.5, breakdown strength 50kV / mm, tensile strength 200MPa, interlayer peel strength 4.2N / cm.

[0129] Example 4 (weighed by weight)

[0130] S1: Raw Material Preparation

[0131] Layer 1: Based on Example 2, add 0.8 parts of PET nanofibers with a diameter of 100nm, 10nm of Al2O3 coating, accounting for 0.8%, and disperse with a screw at 400r / min.

[0132] 4th layer: 0.8 parts of PVDF nanofibers with a diameter of 150nm and a 15nm SiO2 coating accounting for 0.8% were added and dispersed in a static mixer.

[0133] The rest is the same as in Example 2.

[0134] S2-S6 parameters: same as in Example 2.

[0135] Performance data: Dielectric constant 8.9, breakdown strength 49kV / mm, tensile strength 195MPa, peel strength 5.5N / cm, water vapor transmission rate 0.5g / (m²). 2 •24h).

[0136] Example 5 (weighed by weight)

[0137] Differences from Example 1: The second layer of barium titanate has a particle size of 60 nm and a particle size of 6%; the fourth layer of silica has a particle size of 50 nm and a particle size of 5%; the longitudinal stretching is 3.5 times at 122°C, and the transverse stretching is 4.5 times at 135°C.

[0138] Performance data: Dielectric constant 8.6, breakdown strength 47kV / mm, tensile strength 185MPa, interlayer peel strength 3.6N / cm.

[0139] Example 6 (weighed by weight)

[0140] Differences from Example 2: First layer of alumina 150nm, 5%; fifth layer of coating 1.2μm; mold head 210℃; inner air ring 160nm. 3 / h, 18℃.

[0141] Performance data: Dielectric constant 8.7, breakdown strength 48kV / mm, tensile strength 188MPa, interlayer peel strength 3.9N / cm.

[0142] Example 7 (weighed by weight)

[0143] Differences from Example 2: The third layer fiber diameter is 150 nm, the coating is 15 nm, and the thickness is 2.5%; the heat setting crystallization zone is 110°C, and the setting zone is 155°C.

[0144] Performance data: Dielectric constant 8.8, breakdown strength 48kV / mm, tensile strength 190MPa, interlayer peel strength 4.0N / cm.

[0145] Example 8 (weighed by weight)

[0146] Differences from Example 1: The ratio of the fifth coating layer is 2.8:1, and the total film thickness is 60 μm; the corona power is 1.5 kW and 60 m / min.

[0147] Performance data: Dielectric constant 8.5, breakdown strength 46kV / mm, tensile strength 185MPa, interlayer peel strength 3.7N / cm.

[0148] Example 9 (weighed by weight)

[0149] Differences from Example 3: The second layer of barium titanate is 80nm and has a strength of 8%; the fourth layer of silicon dioxide is 100nm and has a strength of 7%; the maximum pressure of the pinch rollers is 2.5MPa, and the lifting speed is 7% per cycle.

[0150] Performance data: Dielectric constant 9.0, breakdown strength 49kV / mm, tensile strength 192MPa, interlayer peel strength 4.1N / cm.

[0151] Example 10 (weighed by weight)

[0152] Differences from Example 3: The fiber content of the third layer is 1.2%, the alumina content of the first layer is 80nm, and the curing temperature of the second stage is 115℃ / 10min.

[0153] Performance data: Dielectric constant 8.9, breakdown strength 48kV / mm, tensile strength 190MPa, interlayer peel strength 4.0N / cm.

[0154] Comparative Example 1 (Interlayer-free nanofibers)

[0155] Missing features: There are no nano-coated fibers in the first and fourth layers, and the rest are the same as in Example 4.

[0156] Performance data: Dielectric constant 8.7, breakdown strength 46kV / mm, interlaminar peel strength 2.1N / cm, significant slippage, water vapor transmission rate 0.7g / (m²).2 •24h).

[0157] Comparative Example 2 (Single-layer structure)

[0158] Missing features: No five-layer structure, only a single layer of polypropylene, containing 5% alumina, no functional layer / coating.

[0159] Simplified process: single screw extrusion, 170℃, no gradient cooling / biaxial stretching, coating curing omitted.

[0160] Performance data: Dielectric constant 4.2, breakdown strength 25kV / mm, water vapor transmission rate 5.2g / (m²) 2 • 24h), tensile strength 100MPa.

[0161] Comparative Example 3 (No Gradient Cooling)

[0162] Missing features: S3 uses a single air ring, 25℃, 150m 3 / h, no temperature difference between the inner and outer air circulation.

[0163] Performance data: dielectric constant 7.8, breakdown strength 38kV / mm, interlayer peel strength 2.5N / cm, high internal stress.

[0164] Comparative Example 4 (without biaxial stretching)

[0165] Missing features: Only longitudinal stretching is 3 times, no transverse stretching, the rest is the same as in Example 2.

[0166] Performance data: dielectric constant 7.0, breakdown strength 35kV / mm, tensile strength 130MPa, molecular disorder.

[0167] Comparative Example 5 (coating cured in one step)

[0168] Missing feature: S6 is directly cured at 150℃ for 25 minutes, without a three-stage curing process.

[0169] Performance data: Dielectric constant 8.5, breakdown strength 42kV / mm, water vapor transmission rate 2.8g / (m²) 2 • 24h), the coating cracked.

[0170] in conclusion:

[0171] 1. The role of layered material design

[0172] Examples 1-10 employ a five-layer heterostructure, with each layer achieving complementary performance through nanoparticle doping and functional coatings:

[0173] The first and fifth layers of polypropylene substrate are doped with nano-alumina / silica, providing both mechanical support and a moisture barrier. In Examples 1-3, the water vapor transmission rate is ≤0.8 g / (m²).2 ·24h);

[0174] The second and fourth layers of polyethylene naphthalate / polyvinylidene fluoride introduce high-dielectric nanoparticles, barium titanate, and silicon dioxide, achieving a dielectric constant of 8.2-9.5, which is higher than the 4.2 of the single-layer polypropylene in Comparative Example 2.

[0175] The third insulating layer is embedded with nano-oxide coated fibers to form a three-dimensional reinforced network, which increases the breakdown strength to 45-50kV / mm, far exceeding Comparative Example 1 (no fibers, 46kV / mm) and Comparative Example 4 (no tension, 35kV / mm).

[0176] Comparative Example 2, due to the lack of a layered design, shows that a single polypropylene cannot balance dielectric properties and mechanical strength, resulting in a comprehensive deterioration of all indicators, confirming that a layered structure is the foundation for performance improvement.

[0177] 2. Regulation of nanoparticle parameters

[0178] Particle size and dosage: In the examples, the nanoparticle size was controlled between 20-200 nm, such as barium titanate in Example 3 with a particle size of 100 nm and a dosage of 10%. This range can maximize the specific surface area and dispersibility of the particles. In comparison, it can be seen that if the dosage is too high, such as exceeding 10%, it will lead to agglomeration, while if it is too low, it will not be able to perform its function. For example, in Comparative Example 1, barium titanate was less than 5%, and the dielectric constant was only 8.7.

[0179] Surface treatment: In Examples 1-10, nanoparticles were treated with silane coupling agents to eliminate interfacial voids through chemical bonds, and the interlayer peel strength reached 3.2-5.5 N / cm. However, in Comparative Example 1, due to the lack of interlayer fiber reinforcement, the peel strength dropped sharply to 2.1 N / cm, and slippage was obvious.

[0180] 3. Collaborative optimization of process parameters

[0181] Multilayer co-extrusion and gradient cooling: In Examples 2-3, the extrusion temperature of each layer is controlled at 160-220℃ using independent screws, and gradient cooling is achieved by using an inner / outer air ring temperature difference of 10-15℃ to avoid stress concentration within the film. In Comparative Example 3, due to cooling by a single air ring, the interlayer bonding force decreases, the peel strength is 2.5 N / cm, and the breakdown strength drops to 38 kV / mm.

[0182] Biaxial stretching and heat setting: Examples 1-10 employed 3-5 times longitudinal stretching + 4-6 times transverse stretching at a temperature of 120-150℃, causing the molecular chains to align and achieving a tensile strength of 180-200MPa. Comparative Example 4, lacking transverse stretching, exhibited disordered molecular chains, resulting in a strength of only 130MPa and deteriorated dielectric consistency.

[0183] Segmented curing process: Examples 1-10 employ a three-stage infrared curing process with stepped temperature increases from 70-160℃ to densify the nano-coating, achieving a water vapor permeability ≤0.8g / (m²).2 • 24h). Comparative Example 5 used one-step curing at 150℃ for 25min. The coating cracked due to rapid solvent evaporation, and the water vapor permeability increased to 2.8 g / (m²). 2 •24h).

[0184] 4. The function of anti-slip design

[0185] Example 4, by adding nano-coated fibers (PET / PVDF fibers) to the interface of the 1 / 4 layer, utilizes the anchoring effect between the fibers and the matrix, achieving an interlayer peel strength of 5.5 N / cm, a 45% improvement compared to Example 2 without fibers. Comparative Example 1, lacking this design, exhibits significant slippage, verifying that interface enhancement is the core solution to interlayer separation.

[0186] Advantages of the present invention:

[0187] Superior performance across the board: The dielectric constant of Examples 1-10 (8.2-9.5), breakdown strength (45-50kV / mm), and tensile strength (180-200MPa) are improved by 50%-126% compared with the comparative examples (4.2-8.7, 25-46kV / mm, and 100-170MPa), respectively, and the water vapor transmission rate is reduced by more than 60%.

[0188] Process stability: Through parameter control such as gradient cooling and step-by-step curing, the performance deviation rate of the examples is ≤5%, while the comparative examples, due to simplified processes (e.g., Comparative Example 3 without gradient cooling), exhibit performance fluctuations exceeding 15%. Environmental adaptability: The moisture-proof coating and nanofiber reinforcement extend the lifespan of the examples by more than 3 times in humid and hot environments, solving the problem of easy aging in traditional capacitor films.

[0189] In summary, this invention achieves a comprehensive improvement in the performance of capacitor films through a three-pronged technical solution of layered material design, nanoparticle control, and synergistic process optimization, with all indicators outperforming existing technologies.

[0190] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A process for preparing a multilayer polypropylene capacitor film, characterized in that, Includes the following steps: S1: Raw material preparation, the base resin, nano-ceramic filler, nucleating agent and antioxidant are respectively formulated into multi-layer raw materials; S2: Multi-layer co-extrusion molding, in which multiple layers of raw materials are melted separately by an independent screw extruder and then combined through a co-extrusion die to form a composite preform; S3: Gradient cooling and shaping, the composite film preform is cooled by gradient cooling through an air ring; S4: Biaxial stretching treatment, which involves longitudinal and transverse stretching of the cooled preform; S5: Heat setting and surface treatment, heat setting is carried out in a segmented heating furnace, followed by corona treatment to improve surface energy; S6: Nano-coating curing, the outer coating is cured in stages using an infrared oven.

2. The preparation process of a multilayer polypropylene capacitor film according to claim 1, characterized in that: In step S1: The material is formulated into three layers: the middle layer is a polypropylene core insulation layer with embedded nano-oxide coated fibers; the outer two layers are a polypropylene substrate layer doped with nano-alumina particles and a polypropylene protective layer with a nano-silica moisture-proof coating on the surface. The particle size of the nano-alumina particles is 50-200 nm, and the addition amount accounts for 3%-8% of the mass of the polypropylene substrate layer; The thickness of the nano-silica moisture-proof coating is 0.5-2μm, and it is composed of silica and alumina in a mass ratio of 2-3:

1. The diameter of the nano-oxide coated fiber is 50-200nm, the coating thickness is 5-20nm, and the fiber content accounts for 1%-3% of the mass of the polypropylene protective layer.

3. The preparation process of a multilayer polypropylene capacitor film according to claim 2, characterized in that: In step S1: Prepare five layers of raw materials; The first layer is a polypropylene substrate layer, doped with nano-alumina particles; The second layer is a polyethylene naphthalate functional layer, doped with barium titanate nanoparticles; The third layer is a polypropylene core insulation layer with embedded nano-oxide coated fibers; The fourth layer is a polyvinylidene fluoride functional layer, doped with nano-silica particles; The fifth layer is a polypropylene protective layer, with a nano-silica moisture-proof coating on the surface; The nano-alumina particles in S1 have a particle size of 50-200 nm and are added at 3%-8% of the mass of the first layer. The nano-barium titanate particles have a particle size of 20-100 nm and are added at a rate of 5%-10% of the mass of the second layer. The particle size of nano-silica is 30-150nm, and the amount added accounts for 4%-9% of the mass of the fourth layer; The fifth layer has a nano-silica moisture-proof coating with a thickness of 0.5-2μm, and is composed of silica and alumina in a mass ratio of 2-3:

1. The thickness ratio of the 5 layers of raw materials is 1-3:2-4:3-5:2-4:1-3, and the total film thickness is 50-100μm; The surface of the nano-ceramic filler is treated with a silane coupling agent to enhance the interfacial bonding force with the polymer matrix; The diameter of the nano-oxide coating fibers in the third layer is 50-200 nm, the coating thickness is 5-20 nm, and the fiber content accounts for 1%-3% of the mass of the third layer.

4. The preparation process of a multilayer polypropylene capacitor film according to claim 3, characterized in that: The preparation method of the nano-oxide coating fiber in the third layer of S1 includes: PAN or PET nanofibers were prepared by electrospinning. Nano-oxide layers are coated onto the fiber surface using atomic layer deposition or sol-gel methods; The coated fibers are dispersed in a polypropylene matrix and the third layer is formed by co-extrusion. The fifth layer of nano-silica moisture-proof coating in S1 is coated by comma roller coating, and then calendered by hot press roller after coating. The pressure of the hot press roller is 0.5-1.2MPa, and the roller surface temperature is 140-160℃.

5. The preparation process of a multilayer polypropylene capacitor film according to claim 4, characterized in that: In S1, the raw materials for each layer also include: Layer 1: 81-95.5 parts polypropylene resin, 3-8 parts nano alumina particles, 1-3 parts nucleating agent, 0.5-2 parts antioxidant; Second layer: 83.5-93 parts of polyethylene naphthalate, 5-10 parts of barium titanate nanoparticles, 1-2 parts of nucleating agent, and 0.5-1.5 parts of antioxidant; 3rd layer: 93-96 parts polypropylene resin, 1-3 parts nano-oxide coating fiber, 2-4 parts nucleating agent, 1-3 parts antioxidant; 4th layer: 80-91 parts polyvinylidene fluoride, 4-9 parts nano silica particles, 1-3 parts nucleating agent, 0.5-2 parts antioxidant; The fifth layer consists of 85.5-98 parts of polypropylene resin, 1-2 parts of nucleating agent, and 0.5-1.5 parts of antioxidant, with a surface coating of 0.5-2 μm thick nano-silica moisture-proof coating.

6. The preparation process of a multilayer polypropylene capacitor film according to claim 5, characterized in that: The nucleating agent is at least one of sorbitol, phosphate ester, or metal salt of organic carboxylic acid; The antioxidant is at least one of hindered phenols, phosphites, or thioesters.

7. The preparation process of a multilayer polypropylene capacitor film according to claim 1, characterized in that: Nano-coated fibers are added to the contact surfaces between layers 1 and 2, and between layers 4 and 5, specifically as follows: The first layer, near the second layer, contains dispersed PET nanofibers. The surface of the fibers is coated with a nano-alumina coating via atomic layer deposition. The PET nanofibers have a diameter of 50-150 nm, the coating thickness is 5-15 nm, and the fiber content accounts for 0.5%-1.5% of the mass of the first layer. The fourth layer, near the fifth layer, contains dispersed PVDF nanofibers. The surface of the fibers is coated with a nano-silica coating using a sol-gel method. The PVDF nanofibers have a diameter of 80-200 nm, the coating thickness is 5-20 nm, and the fiber content accounts for 0.5%-1.5% of the mass of the fourth layer. The nano-coated fiber is added as follows: in the raw material blending stage of the first and fourth layers, the pretreated nano-coated fiber is mixed with the base resin, nano-ceramic filler, nucleating agent and antioxidant, and then dispersed evenly by a screw extruder.

8. The preparation process of a multilayer polypropylene capacitor film according to claim 1, characterized in that: The die temperature of the independent screw extruder in S2 is controlled at 200-240℃; In the multi-layer co-extrusion molding of S2, the extrusion temperatures of each layer of raw material are as follows: Layer 1 and Layer 5: 160-180℃; Layer 2, Layer 3, and Layer 4: 180-220℃; In S2, during the extrusion of the composite preform, the nozzle diameter is ≥1.8cm, and the initial clamping roller pressure is set to 20%-30% of the maximum set value. During the film stretching process, the pressure of the clamping rollers is gradually increased by 5%-10% each time until the maximum set value is reached, which is 1.5-3MPa.

9. The preparation process of a multilayer polypropylene capacitor film according to claim 1, characterized in that: The inner airflow of the S3 wind ring is 150-200 m³ / h. 3 / h, external air circulation volume is 80-120m³ 3 / h, the inner air ring temperature is controlled at 15-25℃, the outer air ring temperature is controlled at 25-35℃, and the cooling line height is maintained at 1.2-1.8m; In the S4 biaxial stretching process, the stretching ratio in the longitudinal direction is 3-5 times and the longitudinal stretching temperature is 120-130℃; the stretching ratio in the transverse direction is 4-6 times and the transverse stretching temperature is 130-150℃. During the S4 stretching process, directional indentation lines are provided at the edge of the preform and in areas requiring directional stretching, so that a pre-oriented structure is formed at the indentation lines. The directional indentation lines are formed by laser etching or mechanical imprinting, with a line width of 0.1-0.3 mm and a spacing of 5-10 mm. In the S5 segmented heating furnace, the temperature of the crystallization zone is controlled at 100-140℃, and the temperature of the shaping zone is controlled at 150-170℃. During the S5 heat setting process, the surface roughness Ra of the hot press roller is ≤0.1μm, the roller surface temperature is controlled by the circulation of heat transfer oil, the power of the corona treatment is 1-3kW, and the processing speed is 50-100m / min.

10. The preparation process of a multilayer polypropylene capacitor film according to claim 2, characterized in that: Step S6 involves the segmented curing of the outer coating using an infrared oven. Specifically, the fifth layer of silica moisture-proof coating is cured in segments using an infrared oven. The specific parameters for curing the nano-coating in S6 are as follows: three-stage curing is carried out in an infrared oven, with the first stage temperature at 70-90℃ and the curing time at 10-15 minutes. The second stage temperature is 110-130℃, and the curing time is 8-12 minutes. The third stage temperature is 140-160℃, and the curing time is 5-8 minutes. The hot air circulation rate of the infrared oven is 2-3 m / s, and the water vapor transmission rate after coating curing is ≤1.0 g / (m²). 2 •24h).

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